U.S. and allied strikes against the Islamic State may have killed as many as 6,000 civilians in 2017, as international forces pushed militants out of strongholds in Iraq and Syria, a watchdog group said Thursday.

"In 2017 the war against ISIS moved into the most densely-populated urban centers controlled by the group, with dire results for civilians," Airwars said in a report summarizing its investigations for the year, which it called the "deadliest yet" for Iraqis and Syrians.

ISIS is an acronym for the Islamic State, the extremist group that drew Western nations into a new Middle Eastern conflict after it captured large swaths of Iraq and Syria in 2014.

Airwars, which investigates allegations of civilian casualties by using social media and other information sources, said that between 3,923 and 6,102 noncombatants were "likely killed" in air and artillery strikes by the United States and its partners in 2017.

The estimate for Iraq and Syria was more than triple that of the year before, Airwars said.

While the Airwars data includes strikes by the United States and partner nations including Britain and France, most of the military activity has been conducted by American forces.

The group's estimate is vastly higher than the figure put forward by U.S. Central Command, which conducts its own investigations of selected U.S. strikes.

According to its most recent public report, Centcom has determined that at least 817 civilians have been killed since the air campaign began in 2014. The command continues to investigate other reported incidents.

The alleged increase in civilian deaths took place as the United States and partner forces conducted major operations to recapture the city of Mosul, the militants' most important stronghold in Iraq, and Raqqa, their onetime capital in Syria.

Islamic State militants frequently positioned themselves among civilians and prevented residents from moving freely in areas they controlled.

While U.S. military officials say they take precautions to prevent civilian casualties, they acknowledge the challenge of avoiding unintended deaths in large-scale operations conducted in densely populated urban areas.

The coalition does "everything within its power to limit harm to noncombatants and civilian infrastructure," Pentagon spokesman Eric Pahon said.

"We regularly issue strike reports and civilian casualty reports, including results of credible and noncredible allegations, as well as those still pending assessment, on our public-facing website for the world to see," he said.

The Pentagon has pushed back against a perception in early 2017, when observer groups reported an apparent surge in civilian casualties, that it had altered rules guiding the U.S. air campaign.

The Islamic State has lost most of the territory it once held in Iraq and Syria, and U.S. airstrikes have decreased significantly. But the militants retain an ability to launch terrorist attacks in both countries and beyond.